LABORATORY 8

PHYLUM  CHORDATA

Part II

OBJECTIVES OF THE LABORATORY

1.         To examine the digestive, urinary , reproductive, respiratory and circulatory systems of the Phylum Chordata.

 DIGESTIVE SYSTEMS

            The gastrointestinal (GI) tract is a tube starting at the mouth and emptying either into a cloaca or to the external environment via an anus.  The ingestion, digestion and absorption of food plus the elimination of undigested wastes are the functions of the GI tract.  The tube of the tract is, in chordates, subdivided into the oral cavity (mouth), pharynx (throat), esophagus, stomach, small intestine and large intestine.  Accessory organs are associated with the tract.  Examples of accessory organs are teeth, tongue, salivary glands, liver, and gallbladder.

            Beyond the esophagus, the digestive organs are located in the coelom.  Within the coelom, most of these organs are suspended by a mesentery extending from the median dorsal wall.  Blood vessels and nerves extend to these organs via the mesentery.  The lumen of the digestive tract can be thought of as continuous with the external environment and even though it contains fluids and enzymes that are released into the cavity, small molecules, which result from digestion, must diffuse across the epithelial cells, which line the GI tract to enter the bloodstream and tissue fluid.  The major products of digestion are monosaccharides, glycerol, fatty acids and amino acids.  These and other small molecules are taken into the internal environment where further processing occurs.

            Caudal to the pharynx, differences in the anatomy of the digestive tract are correlated primarily with the nature and abundance of food, regardless of the animal's environment (i.e. whether aquatic or terrestrial).  Because the walls of plant cells are not digestible by animal enzymes and because they are mixed with the digestible matter, processing takes considerably longer in animals whose diets include large amounts of plant material.  Consequently, the intestine of herbivores is usually longer than those of omnivores and especially carnivores.  Herbivores depend on bacteria and protozoa to break down cellulose and many have a large blind (closed at one end) sac or tube called a caecum at the junction of the small and large intestines where food is held and processed by these microorganisms.  Caeca increase the surface area available and can be found almost anywhere in the digestive tracts of animals.  Since most digestion and absorption occur in the small intestine, a caecum at the end of the small intestine is not the best place for the breakdown of cellulose to occur.  Rabbits have solved this problem of poor location by producing two kinds of feces, one type just from the caecum, which are eaten immediately for additional processing.

            In fish, the mouth opens into the oropharyngeal cavity, which extends to the esophagus.  Both respiratory water and food are taken in through the mouth.  Fish have a primary tongue that is an elevation in the floor of the oropharynx, which is immoveable.  Fish do not have any salivary glands.  In carnivorous fish, such as the perch, the teeth are used to capture and hold the prey so it can be swallowed.

            In mammals, the mouth opens into the oral cavity, which extends to the pharynx.  The palate separates the respiratory and oral cavities enabling breathing and processing of food in the oral cavity to occur simultaneously.  Most mammals have a moveable, protrusive tongue, which is used primarily to manipulate materials in the oral cavity but may also be used in procuring food or water.  Virtually all mammals have salivary glands.  Saliva functions as a lubricant and may contain enzymes, which begin the digestion of carbohydrates.

            Except for baleen whales and anteaters, all mammals have teeth.   The type of teeth a mammal has reflects its diet.  Molars have flat surfaces for crushing and milling and are characteristic of herbivores.  Premolars are used for shearing, cutting, and slicing except in advanced herbivores where they function like molars.  Incisors, chisel-shaped teeth in the front, are useful for biting.  Premolars and incisors are found in herbivores, carnivores and omnivores.  Pointed teeth (canine teeth in mammals) are characteristic of carnivores and are useful for tearing flesh as well as catching and holding prey.

            Because specialization in feeding reduces competition among mammals, differences in dentition are important in identifying and classifying different groups.  In addition, with rare exceptions, each mammalian species has a set number of teeth.